Imaging phantom, or simply phantom, is a specially designed object that is scanned or imaged, especially in the field of medical imaging to evaluate, analyze, and tune the performance of various imaging devices. A phantom is more readily available and provides more consistent results than the use of a living subject or cadaver, and likewise avoids subjecting a living subject to direct radiation.
The image quality in carrying out imaging activities for diagnostic, monitoring or quality control purposes needs to be maintained at acceptable levels in order to prevent misdiagnosis and oversights in the medical field and malfunctioning of equipment in the industrial field. Image quality is defined and quantified in terms of contrast, resolution and noise in the image and these may be assessed quantitatively and qualitatively by routine imaging of suitable phantoms. Routine image quality assurance aids in identification of faulty equipment and hence promotes improved patient treatment or quality control in the industrial field.
As regards the medical field with which this invention is particularly concerned, each different type of equipment is typically provided with its own dedicated type of phantom so that the equipment can be calibrated and maintained so as to be most effective in use. However, such dedicated phantoms are typically proprietary to the manufacturer of the medical equipment concerned and generally have extremely limited application, typically only to that particular item of equipment used for a particular type of medical imaging. As a general rule, the cost of the phantoms is high consequent on limited numbers of production and the proprietary nature of the radiation based equipment concerned.
Different imaging modalities are used in diagnostic radiology in order to diagnose and follow up a variety of disease conditions. In order to ensure that the images obtained are of acceptable quality and can be used clinically for accurate diagnosis, image quality has to be evaluated and maintained. Image quality is a subjective concept that requires certain measures in order to be quantified, by using a phantom for example. Image quality is defined in terms of contrast, spatial resolution and noise by using various inserts, of different shapes, sizes and made from different materials, at fixed locations in the phantom, through visual inspection and by mathematical calculations.
Image contrast is the difference in the gray scales of adjoining regions in an image. It is affected by subject contrast, that is to say, the differences in signals before being registered as part of the image, detector contrast being how the detector converts the signal into output and digital image and display contrast in post-acquisition image processing.
Spatial resolution describes an imaging system's capability to distinguish two closely adjoining structures as separate as they become smaller and closer together, i.e. the amount of detail in the image. It is described by a point spread function (PSF), line-spread-function (LSF) or edge spread function (ESF) and these are used to calculate the modulation transfer function (MTF), which shows the percentage of an object's contrast as a function of its size.
Noise is a random “grainy” appearance in an image. Quantum noise is determined by the number of signals used to form the image and it influences the ability to detect low contrast objects.
The image quality parameters that have to be assessed with x-ray producing equipment in the diagnostic radiology environment therefore include grey scale linearity, circular symmetry, high contrast resolution, low contrast detectability, image uniformity, spatial resolution, image noise, scaling, magnification, distance measurements, contrast-detail relationships and the presence of artifacts, e.g. blurring. These parameters are assessed and quantified in CT scanning, mammography, fluoroscopy and x-ray imaging using a phantom with a variety of inserts. The obtained results are compared to baseline values. When image quality is not maintained, for example when images are blurred, contain artifacts, too much image noise and have poor low contrast detectability, small lesions and abnormalities can be missed.
X-ray photons can penetrate an object without undergoing an interaction, it can be completely absorbed in the object, thus contributing to dose and not to image formation, or it can be scattered. Photoelectric interactions only occur if the photon energy is greater than the binding energy of the electrons. All energy is transferred from photons to atomic shell electrons, which are ejected from the shell with certain energy. The ejected electrons spend their energy in the object, close to the original interaction site. The remaining vacancies are filled by higher shell electrons, producing characteristic x-rays. With Compton scatter a portion of the incident photon energy is absorbed and the photon is scattered through an angle. Contrast, due to differences in atomic numbers in a heterozygous object, depends strongly on the energy of the incident photon beam and thus on the beam kilo voltage (kV). At higher energies, where the photoelectric effect dominates, kV changes will have significant influences on contrast. In low atomic number materials, like the breast for example, lower kV settings are used for optimal contrast. With higher atomic number materials, like bone for example, the kV dependence of contrast is more pronounced over a wider kV range. In projection imaging a shadow image of the internal anatomy is projected on the image receptor, for example general x-rays, fluoroscopy and mammography. In CT scanning image reconstruction, from the photon penetration data, is used for image formation. The photons emerging through a heterozygous object contains an image of the object in terms of differences in attenuation through the different parts of the object. Here contrast is the amount of variation in the x-ray photons between different areas in the image. Hence the contrast is determined by the characteristics of the x-ray photon beam and the composition of the imaged object. Image receptors can either be x-ray film, CR plates or DR detectors.
Currently separate phantoms are used for image quality assurance in each of these imaging modalities. The ACR mammography phantom contains fibres (1.56, 1.12, 0.89, 0.75, 0.54, 0.40 mm in diameter), simulates tumorous masses with 2.00, 1.00, 0.75, 0.50, 0.25 mm diameter hemi-spheres and micro-calcifications with 0.54, 0.40, 0.32, 0.24, 0.16 mm speck groups. It is 4.2 cm thick and consists of 3.5 cm Lucite and a 0.7 cm thick paraffin insert, which contains the image quality indicators.
The NORMI 13 phantom has 7 dynamic steps, consisting of different thickness copper plates from 0.0 mm to 2.3 mm, for evaluation of contrast resolution. For low contrast evaluation, 6 disks with contrasts of 0.8% to 5.6% are visually inspected. It also evaluates grey scales, field size and image uniformity.
The NORMI Rad/Flu phantom incorporates a copper step wedge for grey scale assessment, a resolution test pattern, a grid plate and 8 low contrast detection inserts. Resolutions from 0.6 to 5.0 lppmm are included. Contrast is visually evaluated with a copper step wedge, with 17 steps of thickness 0.00 to 3.48 mm at depths of 13 mm and 5 mm. The phantom also evaluates position verification and distance measurements.
The Catphan phantom has two low contrast modules. The supraslice region has three groups of inserts, each with nine circular objects with diameters between 2 and 15 mm and contrast of 0.3, 0.5 and 1.0%. In the subslice module three groups of four inserts each are contained. Diameters range between 3 and 9 mm and contrast is fixed at 1.0%. The AAPM (American Association of Physicists in Medicine) report no 1 also suggests evaluating slice thickness with ramps placed in the phantom.
In resource limited hospitals and other medical facilities three main problems may be identified. The first of these is the cost of acquiring a number of different currently existing phantoms for each different type of proprietary equipment that needs to be tested or calibrated from time to time. The second problem is that of manpower and expertise as many hospitals and diagnostic radiology units do not have sufficiently well trained personnel such as medical physicists to do routine quality control with dedicated phantoms. In this regard, the results put out by current phantoms are not easy to analyze and interpret and this makes corrective action decisions difficult. Thirdly, there is the problem that conducting image quality result analysis and deciding on corrective action takes time and often needs to be done by qualified staff that are simply not available.
There is a need for an improvement to the present situation.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.